Resto/Balance/Unholy would be better or Resto/Feral/Frost, reasoning is that Unholy is good at spread damage with DoTs on everyone and Balance can do the same, whereas Frost is good at bursting one target and Feral can do the same.

3v3 is about one of two things, either blow CDs and instantly kill one person (the Feral/Frost setup) or play more defensive and lower the health of the entire enemy team and then pop CDs and do the last bit of damage (the Balance/Unholy setup).

That being said, generally having two of the same class is bad and even more so with Druids, Symbiosis is one of the best skills in the game and you won't be able to both use it, however as I said, you can still do decently well with that comp. I'd probably run Balance/Unholy personally, because burst comps are a bit less fun imo and if you don't kill something fast with it you're going to be in trouble haha.

Frost is weak in arena at the moment, it's so easy to control frost DK and he won't do any damage but on other hand unholy overall pressure is just way stronger and you can't stop it's damage Balance/Rdruid/Uh DK is quite weak but if you guys can't play any other comp go for it. I'd say feral/UH would be way better.